Effect of residual natural gas saturation on multiphase flow behaviour during CO2 geo-sequestration in depleted natural gas reservoirs

Hydrocarbon reservoirs, either depleted or active, present a large potential storage volume for geological CO2 sequestration. However, compared to other candidate storage sites e.g. deep saline aquifers, unminable coal seams, etc., in particular, they present a degree of uncertainty associated with potential effects of residual hydrocarbon, left behind during the hydrocarbon production operation, on the sequestration process. This can create a challenge when it comes to modelling the multiphase flow characteristics of the underground fluid-rock system over both short and long term.This paper presents in details the equipment, procedure, protocol and the results of a series of core-flooding experiments conducted to understand the extent of the effects that the existence of residual natural gas saturation can have on the multiphase flow characteristics of the fluid-rock system during CO2 geo-sequestration in sandstone reservoirs. With this knowledge, from the multiphase flow behaviour point of view, some of the uncertainties involved in sequestrating CO2 in depleted natural gas reservoirs can be properly addressed. The experimental results obtained here show that the presence of residual natural gas saturation can have significant impact on the multiphase flow characteristics of the fluid-rock system during CO2 geo-sequestration in depleted gas reservoirs. Despite what some researchers believe, based on the results obtained, injecting CO2 into such geological media may not provide additional secure storage capacity as any additional gas stored in the form of the desired residual saturation easily becomes mobile again putting additional strain on any overlaying seal. Also, the generated experimental data show that storing CO2 into depleted gas reservoirs may suffer from low injectivity during the early stages of the operation, however, the injectivity is expected to improve over time with further injection.

► Residual natural gas strongly influences the multiphase flow during CO2 sequestration.
► Almost all the residual natural gas is recovered through CO2 injection.
► Low injectivity is an issue during CO2 injection in depleted natural gas reservoirs.
► Much of residually trapped CO2 in a depleted gas reservoir may become mobile again.
► Permeability strongly influences multiphase flow during CO2 storage in gas reservoirs.